Thermoplastic films which provide a liquid barrier in addition to providing moisture vapour permeability are known in the art. Particularly preferred are hydrophilic continuous films that do not allow the flow of moisture vapour through open pores or apertures in the material, but do transfer substantial amounts of moisture vapour through the film by absorbing water on one side of the film where the moisture vapour concentration is higher, and desorbing or evaporating it on the opposite side of the film where the moisture vapour concentration is lower. Such films are typically formed from a thermoplastic polymeric composition comprising a thermoplastic hydrophilic polymer, or a blend of thermoplastic hydrophilic polymers. Thermoplastic hydrophilic polymeric compositions having the above described characteristics are also known in the art as “monolithic compositions”, and the moisture vapour permeable, liquid impermeable layers or films made therefrom are known as “monolithic layers” or “monolithic films”.
For example WO 95/16746 discloses films prepared from mixtures of a) block copolyether ester, block copolyether amides (e.g. Pebax™) and or polyurethane and b) thermoplastic polymer which is incompatible with a, and c) a compatibiliser. The films are liquid impermeable and have moisture vapour permeability of about 700 g/m2 day. Also, U.S. Pat. No. 5,447,783 discloses a vapour permeable water resistant multi component film structure having at least three layers. The outer layers are hydrophobic copolyetherester elastomers having a thickness of 1.3-7.6 micrometers and a WVTR of 400-2500 g/m2 24 h and the inner layer is a hydrophilic copolyetherester elastomer having a thickness of 7.6-152 micrometers and a WVTR of at least 3500 g/m2 24 h.
U.S. Pat. No. 5,445,875 discloses a waterproof, bloodproof and virusproof breathable laminate. The laminate comprises a woven/nonwoven fabric and an extruded film such as Hytrel™ having a thickness of about 1 mil (25.4 micrometers).
U.S. Pat. No. 5,532,053 discloses a high moisture transmission medical film which can be laminated onto a nonwoven material. The laminate film comprises a first layer of polyetherester copolymer and second and third layers selected from a specified group of polymers. The film has a MVTR of greater than 750 g/m2 24 h (ASTM F1249) and a thickness of less than 1 mil (25.4 micrometer) preferably 0.6 mil to 0.75 mil (15-19 micrometers).
U.S. Pat. No. 4,938,752 discloses absorbent articles comprising films of copolyether esters which have reduced water permeability, a water vapour permeability of 500 g/m2 24 h (as measured in a specified described test) and a thickness of 5-35 micrometers. There is no disclosure of a supportive substrate.
U.S. Pat. No. 4,493,870 discloses a flexible layered waterproof product comprising a textile material covered with a film of a copolyetherester having an MVTR of at least 1000 g/m2 24 h (ASTM E96- 66) having a thickness of 5 to 35 micrometers.
GB 2024100 discloses a flexible layered water resistant article comprising a microporous hydrophobic outer layer which is moisture vapour permeable but resists liquids and a hydrophilic inner layer of polyetherpolyurethane having a MVTR of above 1000 g/m2 24 h.
In our patent applications WO 99/64077 entitled “Low viscosity thermoplastic compositions for moisture vapour permeable structures and the utilisation thereof in absorbent articles”, and WO 99/64505 entitled “Low viscosity thermoplastic compositions for structures with enhanced moisture vapour permeability and the utilisation thereof in absorbent articles”, thermoplastic hydrophilic polymeric compositions comprising a thermoplastic hydrophilic polymer, or a blend of thermoplastic hydrophilic polymers, are disclosed for making hydrophilic continuous moisture vapour permeable, liquid impermeable films or layers having preferred characteristics of moisture vapour permeability and liquid imperviousness. The disclosed preferred thermoplastic hydrophilic polymeric compositions are also readily processable so as to provide a coating having the desired thickness onto a substrate, so avoiding the need of complex traditional extrusion apparatuses. This is achieved by modifying the viscosity of the thermoplastic hydrophilic polymers by means of the inclusion in the composition of a suitable plasticiser or blend of plasticisers that lowers such viscosity. This allows to utilise with these preferred compositions typical process conditions known in the art for the direct coating of low viscosity hot melt compositions onto a substrate in order to form a moisture vapour permeable, liquid impervious film or layer.
Particularly preferred hydrophilic plasticisers are described in WO 99/64505, which, in addition to adjusting the viscosity of the compositions, also provide the thermoplastic hydrophilic polymeric compositions with a further benefit in terms of moisture vapour permeability.
As shown in the two prior art documents cited above, our thermoplastic hydrophilic polymeric compositions (“monolithic compositions”) are particularly suitable for the manufacture of typically continuous layers or films to be used as such, and which are moisture vapour permeable and liquid impermeable, as well as of composite laminated structures wherein one or more typically continuous layers of the thermoplastic hydrophilic polymeric composition are connected to one or more different substrates, for example a fibrous layer such as a nonwoven fabric, said composite laminated structures typically also being moisture vapour permeable and liquid impermeable. The above compositions are formulated as low viscosity hot melt formulations, and are typically processed, in order to prepare layers or films or laminates, in the molten state, e.g. by known hot melt coating techniques. Alternatively, emulsion or solution processes are also available for similar thermoplastic hydrophilic polymeric compositions as those disclosed in our patent applications WO 01/97870 and WO 01/98399, respectively entitled “Thermoplastic hydrophilic polymeric compositions with low water solubility component” and “Thermoplastic hydrophilic polymeric compositions with high water solubility component”.
While the thermoplastic hydrophilic polymeric compositions disclosed in our patent applications cited above are particularly suitable since they are easily processable and can also provide films and layers, which can also be incorporated into laminated structures having good characteristics of liquid imperviousness and moisture vapour permeability, they can be further improved in order to achieve even better characteristics, namely in terms of mechanical properties.
The above compositions in fact typically comprise relatively high amounts of plasticiser, in order to both adjust the viscosity in the molten state, and also to provide a higher moisture vapour permeability. However, such relatively high amounts of plasticiser can lead to films and layers having mechanical properties which can be too low for certain product applications where high stresses and/or strains can be applied in use. For example, the compositions can be comprised as films or layers as such, or alternatively incorporated in composite laminated structures, in e.g. disposable absorbent articles such as sanitary napkins or pantiliners, or disposable baby diapers. It may happen, in certain usage conditions of said articles, that high stresses are applied to the film, layer or laminated structure comprised in the article, therefore causing possible rupturing of the film or layer, and/or a delamination in the laminated structure, i.e. at least a partial detachment of the thermoplastic layer or film from one substrate.
Addition of suitable tackifier resins to the thermoplastic hydrophilic polymeric composition can provide a better bonding of a film or layer comprising the composition to a substrate, but does not increase the mechanical strength of a film or layer per se made of the thermoplastic hydrophilic polymeric composition.
Moreover, addition of a relatively high amount of tackifier resin can lead to a composition having residual permanent tackiness, which may not be preferred in certain product applications. Addition of some tackifying resins in relatively high amounts can also influence negatively the overall moisture vapour permeability of the resulting film or layer.
It is therefore an object of the present invention to provide a film or layer comprising a thermoplastic hydrophilic polymeric composition, which is liquid impermeable, and has a high moisture vapour permeability, and which also has increased mechanical properties, particularly in terms of a better mechanical strength.
It is a further object of the present invention to provide a liquid impermeable, moisture vapour permeable film or layer which, when comprised into a composite laminated structure, by being suitably bonded to at least a substrate, e.g. a nonwoven, preferably by direct coating onto said substrate, has a better bonding strength, i.e., a higher resistance to delamination under the stresses and strains typically encountered under usage conditions of said laminated structures, therefore providing a stronger composite laminated structure.
It is still a further object of the present invention to provide a liquid impermeable, moisture vapour permeable film or layer having the characteristics as stated above, without a residual permanent tackiness which may be undesirable in certain product applications.
It has been surprisingly discovered that, by including selected suitable functionalized copolymers in the thermoplastic hydrophilic polymeric compositions, comprising selected hydrophilic polymer or polymers and selected suitable compatible plasticiser or plasticisers, a liquid impermeable, moisture vapour permeable film or layer can be obtained from said compositions, which film or layer has an increased mechanical strength, and preferably also an increased bonding strength, when incorporated into a composite laminated structure, particularly when said thermoplastic hydrophilic polymeric composition is applied in the molten state to said substrate, for example by hot melt coating.
Further advantages of the films or layers according to the present invention will be illustrated in the following description and examples.